A known phosphor for a dispersion-type EL is ZnS that is added with an activator such as Cu, Ag or a co-activator such as Cl, I, or Al. This type of phosphors emits large amount of light by ultraviolet light excitation and electron beam excitation and is also used as a phosphor for CRT. For the light emission from the dispersion-type EL, electrons should be efficiently injected by an electric field. It is understood that in the ZnS phosphor for the dispersion-type EL, Cu2S (cuprous sulfide) is precipitated in stacking faults in the ZnS crystal, and an electric field discharges electrons e− or holes h from the electrically conductive Cu2S, which provide the light emission from the ZnS phosphor body.
For the sufficient precipitation of Cu2S in the ZnS particles, a certain size of particles are necessary. Practically, a phosphor is made available that has an average particle size of about 30 μm. This large particle size of the phosphor causes the light emitting layer to have a film thickness of at least 30 μm or more, which increases the operating voltage. In addition, the light emission inside the phosphor particle is absorbed before it is discharged outside, thereby providing insufficient brightness. Particle size reduction to solve the above problems does not provide sufficient stacking faults where the Cu2S is precipitated, thereby reducing the brightness. Therefore, various proposals have been conventionally made.
For example, patent literature 1 proposes formation of a layer on the surface of the ZnS phosphor of an ultrafine particle, the layer being of different conductivity type from the phosphor. Specifically, for example, when the base is of p-type, a layer of n-type is formed on the surface, and when the base is of n-type, a layer of p-type is formed on the surface. This proposal focuses attention on the fact that Cu2S is a p-type semiconductor, and intends to add a similar function on the phosphor surface instead of precipitating Cu2S. However, the change of the conductive type of the phosphor surface needs diffusion of dopants under a high temperature or the like. This suffers from the loss of the performance of the phosphor body and less brightness.
Patent literature 2 proposes a configuration in which a Cu2S layer is formed on the ZnS phosphor surface as shown in FIG. 7. Although patent literature 2 has the same concept as the patent literature 1, literature 2 is different from the literature 1 in that it uses Cu2S that can be easily generated from the Cu-added ZnS phosphor. Because, however, the phosphor surface is covered by electrically conductive Cu2S, a leak current increases the power consumption, thus reducing the light emission efficiency.